1. Field of the Invention
The present invention relates generally to an optical transmission module used in the optical communications field, and more particularly to a mounting structure of an optical transmission module for performing conversion of an optical signal into an electrical signal or conversion of an electrical signal into an optical signal.
2. Description of the Related Art
In the recent information communications field, high-speed large-capacity processing and high-speed data transmission are required in response to the advancement of information. To meet this requirement, optical transmission is indispensable and preparations are now proceeding toward the expansion and diffusion of an optical communications network.
Known as a device used at many sites in an optical transmission system is an optical transmission module having an optical circuit and an electrical circuit in combination for performing opto-electrical conversion or electro-optical conversion. At present, the production scale of the optical transmission module per communications maker is about 105 products per year. However, it is that the production scale required in the future will become one million or more products per year in response to the diffusion of an optical communications network and that the production cost must be reduced to about 1/10 or less of the present level. Accordingly, it is strongly desired to establish any form of the optical transmission module which can realize mass production and low cost by minimizing the number of components to simplify the assembly process and can also ensure high reliability and long service life.
The components mounted on a printed wiring board built in a communications device are generally classified into a surface mount type and a through hole mount type. A typical example of the surface mount type components is an LSI, which has a form called a flat package. Such a component is soldered to the printed wiring board by a reflow soldering process. This process is performed by printing a solder paste on the printed wiring board, making the surface mount type component stick to the printed solder paste, and heating the whole in a conveyor oven to a solder surface temperature of 220.degree. C. or higher.
A typical example of the through hole mount type components is a large-capacity capacitor or a multi-terminal (200 or more terminals) LSI. The multi-terminal LSI has a terminal form called a PGA (Pin Grid Array). Such a through hole mount type component is soldered to the printed wiring board by a flow soldering process. This process is performed by inserting the terminals of the through hole mount type component into through holes of the printed wiring board, and putting the printed wiring board into a solder bath heated at about 260.degree. C. from the side opposite to its component mounting surface.
In mounting an optical module on the printed wiring board by soldering like the surface mount type component or the through hole mount type component, a so-called pigtail type of the optical module with an optical fiber cord is not suitable as the optical module. That is, the optical fiber cord usually has a nylon coating, and the nylon coating has a low resistant to heat at about 80.degree. C., so that it is melted in the soldering step. Furthermore, the optical fiber cord itself invites inconveniences in accommodation and handling at a manufacturing location, causing a remarkable reduction in mounting efficiency to the printed wiring board. Accordingly, to allow a soldering process for the optical module and reduce a manufacturing cost, the application of a so-called receptacle type of optical module is indispensable.
An example of the receptacle type optical module allowing a soldering process is known from 1996 IEICE, General Meeting Proc., C-207 (Ref. 1). In Ref. 1, there is described a receptacle type optical module manufactured by retaining a photoelectric converter and a bare optical fiber with a ferrule on a silicon substrate, next covering the whole with a silicon cap to hermetically seal an optical coupling region, and finally molding the whole with an epoxy resin.
The silicon substrate is formed with a V-shaped groove for positioning the optical fiber and the ferrule, both of which are simultaneously fixed by the silicon cap. A lead frame is fixed by an adhesive directly to the silicon substrate, so that the lead frame forms electrical input and output terminals. A commercially available MU type connector housing is mounted on an optical fiber connecting portion to realize connections and disconnections with another optical fiber. By flow soldering of the lead frame extending from the molded package, the optical module is mounted on a printed wiring board.
Another example is known from 1997 IEICE, General Meeting Proc., C-361 (Ref. 2). In Ref. 2, a V-shaped groove for positioning a bare optical fiber and a ferrule is formed on a silicon substrate as in Ref. 1. The bare optical fiber is fixed to the silicon substrate by a glass plate through a UV curable adhesive, thereby realizing optical coupling between the optical fiber and a photoelectric converter.
An optical coupling region between the photoelectric converter and the optical fiber is sealed by a transparent epoxy resin. The silicon substrate is fixed to a lead frame forming an electrical input terminal, and the lead frame is connected through a gold wire to the photoelectric converter. The whole except an end portion of the ferrule is molded with a resin to form a molded package. An optical connector adapter is mounted onto the molded package to complete an optical module. The optical connector adapter is used to detachably connect another optical fiber to the optical module. By flow soldering of the lead frame extending from the molded package, the optical module is mounted on a printed wiring board.
The principal concern on the optical transmission terminal equipment is a cost reduction. Of the optical transmission terminal equipment, an optical transmission module having an opto-electric conversion function and an electro-optic conversion function are most costly. It is therefore essential to reduce the number of components and simplify the assembly process as well as ensure the high performance, the high reliability, and long service life of the optical transmission module. However, the above-mentioned prior art techniques have the following problems.
The ferrule with the bare fiber used in each of Refs. 1 and 2 has a form such that the very breakable bare fiber projects from the ferrule. Accordingly, the ferrule is difficult to work, and an inconvenience tends to occur in assembling the optical module from the viewpoint of handling. Further, after mounting the ferrule with the bare fiber, a stress tends to be applied to the root of the projected optical fiber (the boundary between the bare fiber and an end surface of the ferrule), so that there is a possibility that the optical fiber may not be endurable against the connection and the disconnection of the optical fiber connector.
The V-shaped groove formed on the silicon substrate consists of two kinds of grooves for respectively receiving the bare fiber and the ferrule. The V-shaped groove for receiving the ferrule is required to have a depth corresponding to at least the radius of the ferrule (at least ten times the depth of the V-shaped groove for receiving the bare fiber). Accordingly, a long period of time for etching the silicon substrate to form the V-shaped groove is required, causing a problem that the shape accuracy of the V-shaped groove is difficult to obtain.
In each of Refs. 1 and 2, the mounting process requires two steps of fixing the ferrule with the bare fiber in the V-shaped groove and next sealing the photoelectric converter and the optical coupling region, thus requiring an increased number of assembly steps. Further, the adhesive for fixing the optical fiber in the V-shaped groove is low in viscosity, so that the adhesive dropped onto a fixing portion in the V-shaped groove tends to flow along the V-shaped groove immediately and contaminate the optical coupling region easily.
In the molding process, a high pressure in the level of 1000 kgf is applied to a subject to be molded in clamping the mold. In the case of a semiconductor laser module, the allowable misalignment between a semiconductor laser and an optical fiber is very exacting as .+-.1 .mu.m or less. Accordingly, when the above-mentioned high pressure is applied to the ferrule, it is very difficult to keep the alignment between the semiconductor laser and the optical fiber. As a result, variations in optical coupling loss in the optical modules manufactured become large, causing a reduction in yield.
Further, since the molding resin is injected into the mold under a pressure as high as 80 kgf/cm.sup.2, a part of the molding resin squeezed out from a spacing between the mold and the ferrule may contaminate an end portion of the ferrule to be connected to the optical fiber connector, causing an increase in connection loss to the optical fiber connector. Further, the optical module described in each of Refs. 1 and 2 has a form such that the optical fiber connector is plugged into the optical module toward its side surface in one direction. The optical fiber connector is connected or disconnected after soldering the optical module to the printed wiring board. Accordingly, in connecting or disconnecting the optical fiber connector, a stress is concentrically applied to soldered portions between the optical module and the printed wiring board via the leads. As a result, there is a possibility of solder separation due to the stress or leads break due to the metal fatigue, causing an electrical contact failure.